The high affinity IgE receptor belongs to a class of receptors which lack intrinsic kinase activity, but activates non-receptor tyrosine and serine / threonine kinases. Receptor engagement triggers a chain of signalling events leading from protein phosphorylation to activation of phosphotidylinositol-specific phospholipase C, an increase in intracellular calcium levels, and ultimately the activation of more specialized functions. We have investigated the role of the and gamma chains in signal transduction. We found (1) that receptor engagement induces immediate phosphorylation of beta (tyrosine and serine) and of gamma(tyrosine and threonine); (2) that this phosphorylation is specific to activated receptors and is reversible upon receptor disengagement; (3) that the gamma chains are critical for signal transduction but that the beta chain does not seem necessary; (4) that phosphorylation / dephosphorylation is a coupling / uncoupling mechanism; (5) that the receptor activates two distinct phosphorylation pathways, one cell-specific and the other not. We are at present trying to identify the role of beta and are studying further the role of receptor phosphorylation in cell activation.